To date, prostate-specific antigen (PSA) has been known as an early diagnostic marker for prostate cancer. However, PSA levels are elevated not only in case of prostate cancer but also in case of benign prostatic disease, and the specificity is insufficient. Accordingly, only 25% of males having PSA levels of 4 to 10 ng/ml are diagnosed as having cancer via a prostate gland biopsy. Further, it is pointed out recent years that prostate cancer is found in 15% of males having PSA levels of 4 ng/ml or less. Since PSA does not reflect the grade of malignancy, a pathological stage of prostate cancer cannot be predicted based on PSA alone. Accordingly, a novel diagnostic marker that compensates for or supplements the drawbacks of PSA is expected.
At present, serum protein profiling, serum anti-p53 antibody, serum caveolin-1, 50 KDa protein, and the like have been reported to be available for distinguishing prostate cancer from benign prostatic hyperplasia, IGFBP-2 and -3, IL-6 and IL-6sR, TGF-β1, urine MMP, and VEGF have been reported to be available for predicting the disease or prognosis of prostate cancer, and serum hK2 has been reported to be available for both thereof. As histological markers that are specific to the prostate gland, the aforementioned PSA and hK2 have been reported, as those that are specific to prostate cancer, DD3 is reported, and as those that are expressed in almost all prostate cancer cases, AMACR and Apolipoprotein-D have been reported. However, there has been no marker except for PSA that has been put to practical use in clinical settings as a serum marker used for deciding an indication of biopsy or evaluating the grade of malignancy. While EPCA-2, which is a serum marker assumed to be prostate-cancer-specific, was reported recent years, the target molecule of EPCA-2 antibody is deduced to be a nuclear matrix protein, although it has not yet been identified.
Under such circumstances, an antibody RM2-recognized antigen has drawn attention recent years (Saito S., et al., RM2 antigen (β1,4-GalNAc-disialyl-Lc4), “A Novel Carbohydrate Marker for Prostate Cancer,” Biotherapy 20: 418-426, 2006). Antibody RM2 is a monoclonal antibody that is established so as to target novel ganglioside DSGb5 (disialosyl globopentaosylceramide) isolated from the disialoganglioside fraction extracted from renal cancer tissue. In the later research process, the antibody RM2-recognized antigen was found to be a novel sugar chain, β1,4-GalNAc-disialyl Lc4 (RM2 antigen), instead of DSGb5 (Saito S, Levery S B, Salyan M E K, et al: Common tetrasaccharide epitope NeuAcalpha2-3Galbeta1-3 (NeuAcalpha2-6) GalNAc, presented by different carrier glycosylceramides or O-linked peptides, is recognized by different antibodies and ligands having distinct specificities, J Biol Chem 269: 5644-5652, 1994; and Ito A, Levery S B, Saito S, et al: A novel ganglioside isolated from renal cell carcinoma. J Biol Chem 276: 16695-16703, 2001). Specifically, antibody RM2 was prepared by immunizing a mouse with a TOS-1 cell line derived from renal cancer having a disialoganglioside which has the same mobility as DSGb5 on thin layer chromatography. However, analysis of a sugar chain of the TOS-1 cell line, which was carried out at a later date, demonstrated that TOS-1 expressed RM2 antigen having the same mobility as DSGb5, instead of DSGb5 (Ito A, et al, 2001). Consequently, a sugar chain that would be recognized by the antibody became different from a sugar chain that was intended at the time of antibody preparation.
RM2 antigen has a very unique hybrid structure of a lacto-series type 1 sugar chain and ganglio-series sugar chain. It is known that a tumor marker CA19-9 for the digestive system having a lacto-series type 1 sugar chain is widely distributed in the epithelium and in the glands and that ganglio-series sugar chains are abundant in cells derived from the neuroectoderm. Accordingly, the present inventors expected expression of the RM2 antigen in prostate cancer, which is derived from the glandular epithelium and neuroendocrine differentiation was clinically observed in the case of prostate cancer. Thus, using antibody RM2, we inspected prostate cancer, which was subjected to radical prostatectomy. As a result, we discovered that antibody RM2 would reflect the grade of malignancy and react with prostate cancer and that the reactivity of antibody RM2 to benign glands was weak or negative (U.S. Patent Application No. 2005/0221397; Saito S, Egawa S, Endoh M, et al: RM2 antigen (beta-1,4-GalNAc-disialyl-Lc4) as a new marker for prostate cancer. Int J Cancer 115: 105-113, 2005). Samples of radical prostatectomy were formalin-fixed, and most glycolipids, such as gangliosides, were eluted. Thus, antibody RM2 was considered to react with a glycoprotein. However, whether or not RM2 antigen is expressed on a glycoprotein that reacts with antibody RM2 in prostate cancer tissue or serum of a prostate cancer patient was not confirmed, and a marker that is recognized by antibody RM2 was not identified.